Drone pilot Manish Maharjan has helped rescue teams in Nepal find people missing after last week’s floods.

He sometimes used speakers mounted on the drones to call out or play music for those stranded on hillsides.

But the search for hundreds of workers believed to be trapped in power plant tunnels is a different challenge.

Maharjan said the tunnels were filled with mud, like toothpaste in a tube, with no empty space inside.

He joined the Nepal Army’s rescue efforts a day after the Aug. 26 flash floods, which killed more than 1,000 people and left thousands missing in Nepal and Tibet.

The floods, caused by a glacial collapse, damaged roads, buildings, bridges and hydropower plants.

At least 900 workers are missing from 12 hydropower projects buried under mud and debris.

About 500 of them are believed to be trapped inside tunnels.

Some workers managed to call for help when the disaster struck, according to Mohan Kumar Dangi, president of the Independent Power Producers’ Association.

It is unclear whether rescuers have had contact with them since.

Maharjan has been flying thermal drones in the tunnels.

He said searches are hard because of difficult access and the lack of empty space.

He can only fly drones in long, straight tunnels, and bends or zigzags make it much harder.

At the Rasuwagadhi hydropower project, he sent a drone to the end of a tunnel but found no sign of life.

He said it was filled with mud and debris, and he could only imagine the force of the flood.